Multi-drug resistance (MDR) is due to the expression of Pgp (P- glycoprotein), by the MDRl gene, an ATP-driven membrane-bound multi-drug transporter. Recently we have developed synthetic routes to new MDR reversal agents, stipiamide, and a more potent, non-toxic compound 6,7- dehydrostipiamide (DHS, fig.1) that restores the cytotoxicity of adriamycin to resistant human breast cancer cells (MCF-7adrR) at very low concentrations (4 nM). We will now investigate new compounds possessing non-natural spacers, in combinatorial libraries, and benzophenone photolabels. Non-natural polyenes will be produced to identify the optimal structural requirements for MDR reversal using assays with resistant cancer cell lines and purified Pgp. The polyene region of stipiamide will be changed to minimize toxicity and increase MDR reversal. Bi-directional cross coupling reactions with para- disubstituted benzenes will be investigated to selectively attach the two ends. Competition studies with known radiolabeles will be used to verify the direct Pgp binding. The toxicity of each compound will be determined using normal cancer cells and resistant cell lines with added cancer drug. Alternative routes to the anti-1,2-hydroxy methyl will be investigated with chiral auxiliaries and catalytic aldol approaches. The functionality at the two ends and the stereochemistry will be probed using combinatorial libraries. The conditions of the coupling will be optimized at 1:1 iodide:acetylene stoichiometry and applied to the combinatorial libraries. The compounds will be assayed as mixed pools using the MCF-7adrR MDR assay. Specific residues on Pgp will be identified, for the first time, using high-efficiency benzophenone- stipiamide photoaffinity labeler. Microsequencing will be used to identify specific residues found in the binding site. Bi-functional dual-domain compounds will also be made by linking two MDR reversal compounds through the terminal amide. Reversal assays with these compounds will be used to establish the distance constraints between Pgp transmembrane helices TM6 and TM12 that have been implicated to contain the drug and reversal agent binding sites. Results from the libraries, the bi-functional compounds, and the photolabels will establish, for the first time, the location, the binding characteristics, and the distances between the binding bites on Pgp. Data obtained from these experiments will have direct bearing on the 3D-structure of Pgp. This is an important first step toward a molecular understanding of the unique recognition properties, transport mechanism of Pgp, and will further facilitate the design of new, more potent reversal agents.